Digestive System Overview

Overview of Digestion

• The digestive system contains 2 main groups of organs; the alimentary canal and accessory digestive organs

Alimentary Canal

• Responsible for nutrition
• Contains:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine

Accessory Digestive Organs

• Contains:
  • Teeth
  • Tongue
  • Gall bladder
  • Salivary glands
  • Liver
  • Pancreas

Pharynx and Oral Cavity

• The first part of the digestive tract is the pharynx
• The lips and cheeks bound it and contain the teeth and tongue
• The hard palate, composed of the maxillary and palatine bones, is superior to the oral cavity
• The soft palate is posterior to the hard palate
  • It is a muscular tissue that moves during swallowing
• The uvula is a conical structure hanging from the soft palate

Areas of the Pharynx

• There are 3 areas:
  • Oropharynx
  • Nasopharynx
  • Laryngopharynx

Oropharynx

• Extends from the soft palate to the epiglottis
• Contains two sets of tonsils: palatine and lingual
• Tonsils remove pathogens entering the pharynx with their lymphocytes

Nasopharynx

• Located superior and posterior to the soft palate
• Pharyngeal and tubal tonsils are contained within the nasopharynx

Laryngopharynx

• Inferior to the epiglottis and posterior to the larynx
• Opens into the esophagus and larynx

Mouth and Associated Organs

• Food enters the gastrointestinal (GI) tract
• While in the mouth it is chewed, manipulated by the tongue, and moistened with saliva
• The tongue is skeletal muscle
• It mixes food with saliva into a compact mass called a bolus
• The lingual frenulum attaches the tongue to the floor of the mouth allowing movement

Salivary Glands

• Found on the inside of the cheek are two muscles during dissection

• The digastric muscle opens the jaw

• The masseter muscle closes the jaw

• The masseter inserts on the mandible

• The large parotid gland is superficial to part of the masseter and anterior to the ear

  • It produces salivary amylase (ptyalin) and is the largest of the salivary glands

• The submandibular gland (submaxillary) sits anterior to the parotid gland

• The submaxillary duct empties this gland

  • It runs on the lateral aspect of the digastric muscle
  • This carries saliva into the angle of the lower jaw

• The sublingual gland is on the submaxillary duct

Teeth

• Made of similar materials to bone
• Three major components:
  • Hydroxyapatite: Ca5(PO4)3(OH)
  • Bone collagen
  • Cells
• The pH of the oral cavity is around 7.2
• Acids in the mouth come from stomach acid during vomiting, from foods, or from waste products of mouth bacteria

Tooth Anatomy

• Enamel: The hardest substance in the body

• Pulp Cavity: contains arteries, veins, and nerves

• Alveolus: Made of alveolar bone

• Root: made of dentin

• Gingiva: gum

• Periodontal membrane: periosteum is found around the tooth

• Cementum: material that holds the tooth in the alveolus

• Incisors are chisel-shaped for nipping food

• Canines are cone-shaped for tearing

• Premolars function

• Molars function for grinding food

• A permanent dentition has 32 teeth

• 20 teeth make up a deciduous dentition

Digestion Process

• Digestion happens in 6 steps:
  • Ingestion
  • Propulsion
  • Mechanical Digestion
  • Chemical Digestion
  • Absorption
  • Defecation

Propulsion

• Accomplished via peristalsis
• Peristalsis is alternate waves of muscular contraction and relaxation in the primary digestive organs
  • The result squeezes food from one part of the system to the next

Mechanical Digestion

• Physical preparation of food for digestion
• Segmentation mixes food in the intestines with digestive juices

Chemical Digestion

• Carbohydrates, fats, and proteins are broken down by enzymes

Absorption

• Transfer’s digested food into the blood from the digestive canal

Defecation

• Removal of waste products from the body

Swallowing

• Initiated by a reflex
• The mouth closes and the soft palate pushes superiorly to close the nasal passages
• A sphincter valve closes off the eustachian tubes
• The glottis closes, respiration stops, and the glottis bends to close the entrance into the larynx
• Pressure from the food opens the esophagus
• Food then enters the esophagus

Chewing

• Aids the digestive process
• It mixes food with digestive enzymes in saliva
• Increases surfaces area on food
• Makes moving the food easier

Saliva

• Contains ptyalin or salivary amylase

  • Enzymes that break down starches
  • They are only active under certain pH conditions

• A pH of 7.2 is the approximate value of the oral cavity

  • Slightly alkaline

• When the swallowed food reaches the stomach, the pH drops to 3 (very acidic)

  • The ptyalin is no longer active

• Once food is swallowed, smooth muscle in the esophagus carries the bolus by peristalsis

• Bolus enters the stomach by passing through the gastroesophageal sphincter

  • Held shut via muscle contraction

• The lesser curvature of the stomach is anchored to the liver with the lesser omentum

  • It cannot move

• The esophagus and duodenal ends are anchored

  • The stomach can sag on the left side as it fills with food

• Walls of the stomach contains 3 muscle layers

• These muscles do not contract in sync

  • Enables churning and mixing of food in the body of the stomach

• The stomach mixes with water, hydrochloric acid (produced in the stomach), and pepsin

  • This is called chyme

• The pyloric sphincter regulates the release of the chyme

• The stomach also has longitudinal folds within the lumen and are called rugae

  • Increases the surface area of the stomach

Small Intestine

• Made of 3 parts:
  • Duodenum: 10 inches long
  • Jejunum: 8 feet long
  • Ileum: 11 feet long
• The small intestine produces 7 enzymes
  • 17 enzymes total are dumped into the duodenum for digestion
• Most digestion occurs in the small intestine
• 74% of nutrient absorption occurs here
• Arteries, capillaries, and veins are within the plicae circularis
  • The veins drain into the hepatic portal system which ultimately drains into the liver and inferior vena cava
• There is also lymphatic drainage via lacteals which drain into the cisterna chyli
  • These drain into the thoracic duct
• Segmental peristalsis occurs in the small intestine
  • Results in a sausage appearing structure
• The ileocecal valve (sphincter) regulates flow into the large intestine
  • Prevents backflow from the large intestine into the small intestine

Liver

• Functions:
  • Detoxification of blood
  • Carbohydrate metabolism -Glycogenesis: Formation of glycogen from excess glucose in circulation -Glycogenolysis: Breakdown of glycogen in times of fasting -Gluconeogenesis: Formation of glucose in hepatocytes from raw materials
  • Lipid metabolism
    • Synthesizes large quantities of cholesterol and phospholipids
    • Oxidizing triglycerides to produce energy
  • Protein synthesis
  • Secretion of bile
• Bile contains bile salts, water, pigments, cholesterol, and lecithin (a phospholipid)
• Bile salts act like detergents and emulsify fats
  • Makes fat form into small droplets that are more soluble
  • Increases surface area making it more digestible
• Bile is stored in the gall bladder
  • Concentrated here
  • When fat is detected in the duodenum, the gall bladder contracts and bile is discharged into it
• The common bile duct comes into the first inch of the duodenum
  • The opening is called the ampulla of Vater and is controlled by the sphincter of Oddi
  • The sphincter relaxes when the gall bladder contracts

Pancreas

• It produces around 10 enzymes that are responsible for digestion
• These enzymes travel via the pancreatic duct directly into the common bile
• Sometimes it empties directly into the duodenum (anatomic variance)
• Bicarbonate is secreted to neutralize the duodenal contents
• The islets of Langerhans produce insulin and glucagon

Large Intestine

• 5 feet long

• The cecum extends as a 2.5-inch blind sac caudally from the sphincter

• The appendix is off the cecum

  • No purpose for the appendix
  • Some research is showing it may affect an immune function

• Fromm the cecum travels to the ascending colon (5 inches)

  • Retroperitoneal

• The right colic flexure leads to the transverse colon (15 inches)

• The left colic flexure leads to the descending colon (10 inches)

  • Retroperitoneal

• the descending colon leads to the sigmoid colon

  • The name changes at the sigmoid flexure

• The rectum (5 inches) leads to the anal canal (7 inches)

• The final sphincter in the tract is the sphincter ani

• The sigmoid and rectum are also retroperitoneal

• The transverse colon hands on a piece of the mesocolic ligament

  • It hangs into the umbilical region

• The longitudinal bands of muscle in the colon are three bands that do not completely surround the structure

• The only part of the large intestine that has complete muscle coverage is the rectum

• The muscles act like a drawstring forming little pouches in the colon called haustra

• Bacteria live in the colon, synthesizing of Vit. B12 and K, and destroying the "bad" bacteria

• E. coli is an example

• The sphincter ani is an involuntary smooth muscle

• The defecation reflex is kept in control by the sigmoid flexure and peristaltic activity

• The sphincter ani relaxes during peristalsis

  • An external sphincter (skeletal muscle) can oppose this
  • Allowing an individual to “hold it in”